The concept that serum lipoproteins can act as regulators of selected cell functions has only recently been explored. Among the various regulatory phenomena which appear to be mediated by either natural or inducible serum lipoproteins is the suppression of lymphocyte proliferation. While humoral immunoregulatory factors in general are only one of the means by which the character of the host's immune response can be influenced, there exists ample evidence to suggest that they can be operative both as "natural repressors" and as disease or environmentally associated "inducible suppressors". In recent publications, I have described a normal human serum low density lipoprotein (LDL-In) with potent immunoregulatory properties. I now propose to characterize in depth, the phenomenon of LDL-In mediated immunosuppression. Specific areas of study will involve 1) the isolation and biochemical characterization of the biologically active molecule; 2) measurement of the level and specific activity of LDL-In in normal and disease subjects; 3) investigation of the mechanism of action of LDL-In mediated immunoregulation and 4) documentation of the physiological significance of this form of lymphocyte regulation both in vivo and in vitro.